This is a remainder of the previous refactoring series. into_template() could be
implemented as a non-extension method, which allows us to get rid of .clone()
from Literal property extraction. However, there wasn't measurable difference.
Let's not try to overly optimize things. It's probably simpler to switch to
Rc<str> if .clone() really matters.

I'll add a few more optional property types, and I don't want to duplicate the
error message. Type names are capitalized for consistency.

In order to port "branch list" to template, we need to somehow represent
revset.count_estimate() result as a template property. I'm going to add
SizeHint template type for that, and its .upper() and .exact() methods will
have to return optional integers.

Fortunately, the Integer type has no implicit conversion to bool, so
"if(optional_integer, ..)" is not ambiguous.

We'll probably add binary comparison operators at some point, but this patch
also adds size_hint.zero() method. Otherwise, we'll have to write
"if(x.upper() && x.upper() == 0, ..)" to deal with None.

The resulting "branch list" template will look like:
```
separate(", ",
  if(!ref.tracking_ahead_count().zero(),
    if(ref.tracking_ahead_count().exact(),
      "ahead by " ++ ref.tracking_ahead_count().exact() ++ " commits",
      "ahead by at least " ++ ref.tracking_ahead_count().lower() ++ " commits")),
  if(!ref.tracking_behind_count().zero(),
    if(ref.tracking_behind_count().exact(),
      "behind by " ++ ref.tracking_behind_count().exact() ++ " commits",
      "behind by at least " ++ ref.tracking_behind_count().lower() ++ " commits")),
)
```